Swimming is one of the most popular sports, but swimming drowning events often occur due to lack of timely detection and rescue of a drowning man. In order to decrease a possibility of the swimming drowning events, researchers have designed a plurality of drowning alarm systems, for example, drowning alarm systems based on ultrasonic detection, radar detection, camera monitoring, posture detection and acceleration detection. The above-mentioned systems determine whether or not there is drowning event mainly according to some gesture and speed characteristics of swimmers experiencing drowning. However, for drowning alarm systems based on gesture determination, false alarm events can be triggered by non-drowning event (for example, swimmers play in a swimming pool). In addition, some drowning alarm systems require the swimmer to wear a detection device such as a bracelet and a ring. Due to the wearing of the bracelet, the ring or other detection devices, not only the swimming experience of the swimmer can be affected but also the false alarm events caused by the dropping of the detection device (for example, the bracelet) can be occurred. The false alarm events not only make a rescuer fatigue but also affect the rescue of the drowning man truly in need. Therefore, a drowning alarm system with false alarm prevention function is needed.